In the art of lithographic printing, it is desirable to produce a multi-color proofing image to assist the printer in correcting a set of color separation films prior to using them to produce expensive metal based lithographic printing plates. The proof should reproduce the image and color quality that will ultimately be obtained during the printing process. The proof must be a consistent duplicate of the desired halftone image. Visual examination of a color proof should show the color rendition to be expected from press printing using the color separations as well as any defects on the separations which might need to be altered before making the printing plates.
There are two general types of color proofing films, namely the overlay type and the single sheet type. In the overlay type of color proofing, separate transparent supports are used for bearing each color separation partial image. Several of these supports carrying images of the corresponding colors are then superimposed upon each other over a white background sheet to produce a color proofing composite. A key advantage of overlay proofs is that they can be made quickly and can serve as a progressive proof by combining any two or three colors in register. However, overlay proofing has the disadvantage that the superimposed plastic supports tend to darken the color proofing sheet and the impression of the color proofing composite thus prepared is different from that of copies actually obtained with conventional printing presses and with proof presses. Examples of overlay approaches are in U.S. Pat. Nos. 3,136,637; 3,211,553; and 3,326,682.
In the single sheet type of color proofing, a colored image is prepared by successively producing images of different colors on a single receiver sheet. This is done by sequentially applying colored photosensitive layers to a single opaque support. This method more closely resembles the actual printing process and eliminates the color distortion inherent in the overlay system. Examples of such single sheet approaches are found in U.S. Pat. Nos. 3,671,236; 4,260,673; 4,366,223; 4,650,738; 4,656,114; and 4,659,642. Various methods of producing the images on the proofing films are known. These include photosensitive layers whose images are formed either by using a wet developer or by peel apart development. The present invention relates to an improved single sheet type color proofing film of the peel apart type.
U.S. Pat. No. 3,574,049 provides a thermal transfer process for printing a design on a final support by printing a design onto a temporary support, superimposing the temporary support and a final support, applying heat and/or pressure to the superimposed structure and separating the temporary support from the final support which retains the printed design. The affinity of the matter of the design towards the final support is greater than its affinity towards the temporary support This process has the disadvantage of requiring a printing step. Other processes for producing single sheet color proofs of an image embodying thermal transfer and photopolymerization techniques are shown in U.S. Pat. Nos. 3,060,023; 3,060,024; 3,060,025; 3,481,736; and 3,607,264. In these processes, a photopolymerizable layer coated on a support is imagewise exposed through a color separation film. The surface of the exposed layer is then pressed into contact with an image receptive surface of a separate element. At least one of the elements is heated to a temperature above the transfer temperature of the unexposed portions of the layer. The elements are then separated, and the thermally transferrable, unexposed, image areas of the composite transfer to the image receptive element. If the element is not precolored, the tacky unexposed image may now be selectively colored with a desired toner. The colored matter preferentially adheres to the clear unpolymerized material.
U.S. Pat. No. 3,721,557, a method for transferring colored images is shown which provides a stripping layer coated between a photosensitive element and a support. When the photosensitive layer is exposed to actinic light and developed, the more soluble portions are selectively removed to produce a visible image. The image carrying support is pressed against a suitable adhesive coated receptor member and the carrier support is subsequently stripped to accomplish the transfer of the image A fresh layer of adhesive must be applied to the receptor for each subsequent transfer. U.S. Pat. No. 4,596,757 provides a method for transferring images or solid colors which are subsequently imaged. The photosensitive material comprises a carrier support having sequentially disposed thereon a release layer; a colored, photopolymerizable layer; and an adhesive layer. The material can undergo exposure, lamination to a temporary support, wet development, and then lamination to a receptor sheet. Alternatively, the photosensitive material can be laminated to a receptor sheet, undergo exposure, and then wet processed. Both processes require development in an aqueou medium.
In U.S. Pat. No. 4,489,154, a process is claimed which produces a single layer color proof without wet development. The photosensitive material comprises a strippable cover sheet; a colored photoadherent layer; a nonphotosensitive organic contiguous layer; and a sheet support. The material is exposed and peel developed. The positive or negative image is transferred to a receiver base. A fresh layer of adhesive is applied to the receptor for each subsequent transfer.
U.S. Pat. No. 4,895,787 demonstrates a positive-acting peel apart color proofing method which uses a photosensitive element having a transparent support, adhesion promoted surface and a photosensitive composition layer on the adhesion promoted surface. The photosensitive layer comprises a binding resin, colorant, and a photopolymerizable composition. An adhesive layer is adhered to the photosensitive layer. In image formation, a receiver base is laminated to the adhesive layer and then the photosensitive composition is exposed through the transparent support. After peeling apart the support and receiver base, the adhesive layer and the image-wise nonexposed portions of the photosensitive composition are transferred to the receiver base while the imagewise exposed portions remain on the adhesion promoted surface of the support.